Many types of separation processes are known to have existed in the prior art. An example of one is set forth in German Auslegeschrift 1,263,641, which relates to the recovery of metals from old cables and cable waste. The waste material is ground and subjected to magnetic separation; the non-magnetic fraction is thereafter separated by specific gravity in a primary float and sink apparatus, into a specific light fraction, which principally contains the insulation material, and a specific heavy fraction, which contains the non-ferromagnetic metals, for instance, copper, aluminum and lead. The separating medium is a suspension the specific gravity of which is approximately 2.0. The non-ferromagnetic, mainly non-ferrous metal scrap recovered from this primary specific-gravity separation, is also separated by specific gravity in a secondary float and sink apparatus into a fraction of intermediate specific gravity, mainly consisting of aluminum alloys, and a specific heavy fraction consisting of copper, copper alloys and lead. In this secondary separation, the separating medium is a suspension having a specific gravity ranging between 2.8 to 3.0. This process, of course, is also applicable to waste products of other origin, such as discarded cars, refrigerators and other household appliances and other, domestic waste material.
Such well known separation processes, however, exhibit certain disadvantages. The most common is that for every separating step, a heavy medium of a different specific gravity is required, and each medium must be very carefully maintained. Furthermore, a float and sink separation method is not suitable for particles smaller than 6 mm, which are practically always present in an appreciable amount in such waste scrap.
The present invention is directed to a separation process in which only one heavy medium is required and which is capable of separating particles present smaller than 6 mm, down to about 0.5 mm.
According to the present invention, the non-ferromagnetic metal scrap is separated by specific gravity with the aid of a heavy, preferably aqueous, suspension having a specific gravity of at least 2.2. This is accomplished in at least one secondary hydrocyclone whose top angle of the cone-shaped part amounts to 45.degree. to 90.degree. and the scrap is separated into a specifically heavy fraction, mainly comprised of metals having a specific gravity of more than 3.0, which is discharged along with the suspension medium at the tip of the hydrocyclone, and a specifically lighter fraction, mainly comprised of metals having a specific gravity of less than 3.0, which is discharged along with the suspension medium at the overflow of the hydrocyclone. Thereafter, the heavy and lighter fractions are suitably separated from the suspension as by draining. The said non-ferromagnetic metal scrap is mostly comprised of non-ferrous metals, but may contain non-ferromagnetic ferrous alloys, such as certain types of stainless steel.
In the process according to the present invention, use is made of the principle that solids having different specific gravities within a mixture may be separated by specific gravity with the aid of a separating-medium under the influence of centrifugal force. To this end, as a rule, use is made of hydrocyclones. In the separation by specific gravity in a hydrocyclone, the specific gravity of separation may be considerably larger than the specific gravity of the separating-medium, depending on the dimensioning of the cyclone and on the operating conditions.
Usually, hydrocyclones of the type specified in the above are used for separating material consisting of particles having a size of no more than about 50 mm, and the hydrocyclones are mostly dimensioned to have a vortex finder, the internal diameter of which is about twice that of the tangential feed pipe, and a tangential feed pipe the internal diameter of which is about one fifth of that of the cylindrical part of the cyclone; for the separation of particles up to 50 mm an internal cyclone diameter of about 600 mm is often applied.
In the application of the process according to the present invention, it appeared that in the scrap to be treated particles having a size of more than 50 mm, up to about 70 mm, were sometimes present. The separation of particles of this size in a cyclone having the ratios of the several diameters of component parts thereof as mentioned above would require a cyclone having an unduly large diameter (about 900 mm) and would entail an unduly large circulating flow of suspension, e.g., twice the flow required by the smaller cyclone.
It was surprisingly discovered that scrap-particles in size up to 70 mm could be separated in a cyclone having a diameter normally dimensioned for particles up to only 50 mm. This discovery made the use of hydrocyclones technically and economically feasable. For the smaller cyclone to function properly, it is only necessary that the ratio of the internal diameter of the vortex finder to that of the tangential feed pipe of the secondary cyclone be less than 1.5, e.g. 1.4 and that the ratio of the internal diameter of the feed pipe to that of the cylindrical part of the secondary cyclone be between 0.22 and 0.28, e.g. 0.25. Thus, when these ratios are applied in the design of the cyclone, a normal cyclone diameter of 600 mm can be used wherein particles up to about 70 mm can be treated and the flow of the suspension medium can have the normal value for a 600 mm cyclone.
In many cases separation into heavy alloys, such as copper alloys, and lighter alloys proves insufficient, for example, when the lighter alloys mentioned mainly comprised of aluminum-containing alloys having high silicon and magnesium contents. Metallurgical processing of such a mixture for the purpose of recovering the aluminum therefrom is difficult. It is then desirable that the lighter fraction be further separated into a fraction mainly consisting of aluminum, and a fraction comprised of alloys which are specifically lighter than aluminum. To this purpose, according to the present invention, the specifically lighter fraction, mainly comprised of metals having a specific gravity of less than 3.0, is supplied along with a suspension medium of the same composition as used in the secondary hydrocyclone to at least one tertiary hydrocyclone whose top angle of the cone-shaped part amounts to 15.degree.-25.degree.. A middle metal fraction mainly comprised of aluminum is discharged along with the suspension medium at the tip of the hydrocyclone and a lightest metal fraction mainly consisting of lighter metals and alloys is discharged along with the suspension medium at the overflow of the hydrocyclone. Thereafter, these middle and lightest metal fractions are separated from the suspension medium through a draining process.